For many years conventional magnetic storage devices have been used to store data and information. Magnetic storage devices generally include units (“bits”) of magnetic material that can be polarized to distinct magnetic states, such as a positive state and a negative state. Each bit can store information (generally binary information in the form of either a 1 or a 0) according to the magnetic polarization state of the bit. Accordingly, magnetic storage devices generally include a “read” element that passes over the magnetic material and perceives the magnetic polarization state of each bit and a “write” element that passes over the magnetic material and changes the magnetic polarization state of each bit, thereby recording individual units of information. Therefore, the amount of information that can be stored on a magnetic storage device is directly proportional to the number of magnetic bits on the magnetic storage device.
There are various types of magnetic storage devices and each type involves different fabrication processes. For example, conventional granular magnetic recording devices are disks that have magnetic layer bits with multiple magnetic grains on each bit. In granular magnetic devices, all of the bits are co-planar and the surface of the disk is smooth and continuous. In order to increase the amount of information that can be stored on a granular magnetic disk, the bit size can be decreased while keeping the grain size the same. However, with smaller bits there are fewer grains on each bit, which decreases the signal to noise ratio (less signal, more noise). In order to maintain a better signal to noise ratio, methods have been developed that decrease both the bit size and the grain size, thus keeping the same number of grains on each bit. However, when the grains become too small, thermal fluctuations can cause the grains to spontaneously reverse polarity, thus resulting in unstable storage and a loss of information.
Bit-patterned media devices are another example of magnetic storage devices. In bit-patterned media, the bits are physically etched into a surface using conventional lithographic and etching techniques. In contrast to granular magnetic recording devices, bit-patterned media devices are topographically patterned with intersecting trenches and elevated bit islands. In some instances, the trenches are etched directly into a magnetic material and in other instances the physical patterns are etched into a substrate and then a magnetic material is coated over the patterned substrate. Because of the physical separation between the elevated bit islands and the trenches, the width of each distinct bit island can be decreased in order to increase the areal bit density of the device while still maintaining a high signal to noise ratio and high thermal stability.
In fact, the areal bit density of conventional bit-patterned media is limited only by the resolution limits of conventional lithographic techniques and not by stability issues like in granular magnetic recording devices. For example, bit-patterned magnetic recording media may be thermally and magnetically stable at bit densities of greater than one trillion bits per square inch (Tbit/in2). However, conventional lithography can only generate bit pattern densities and associated servo features up to about 0.5 Tbit/in2.
Lithography involves coating a medium to be patterned with a resist material and emitting radiation (e.g. light or electron beams) across the resist material in a certain pattern to develop certain portions of the resist material. This radiation prepares portions of the resist material to be selectively removed. The removed portions of the resist material expose a patterned portion of the medium for further processing. While conventional lithographic processes can pattern features as small as about 30 nanometers, the demand for even smaller features and higher density patterns in bit-patterned media has pushed conventional lithographic techniques to their resolution limits.
Accordingly, other techniques have been and are currently being developed to increase the pattern density of bit-patterned media. For example, directed self-assembly of block copolymers has been implemented for achieving bit densities of greater than 1 Tbit/in2. As further described specifically in U.S. Pat. No. 8,059,350 and U.S. Pat. No. 8,119,017, both of which are assigned to the same assignee as this application, directed self-assembly of block copolymers can be used to multiply the areal bit density and/or to rectify the conventional lithographic bit pattern. Additional details and description relating to self-assembly of block copolymers is included below in the detailed description as it pertains to the subject matter of the present disclosure.
However, producing patterned media using density multiplication techniques, such as directed self-assembly of block copolymers, can limit the patterned media to a single feature density. In other words, if a certain application requires a patterned medium that has both a high feature density region and a low feature density region, conventional density multiplication techniques may not be used. For example, conventional bit-patterned magnetic recording hard disk drives generally include data regions and servo regions. The data regions generally include repeating bit islands for reading/writing information and the servo regions, in part, are for position error signaling (PES). In order to improve the storage capabilities and performance of a bit-patterned magnetic hard disk drive, it may be beneficial to increase the data bit island density beyond the resolution limits of conventional lithography while still maintaining the servo feature density within the resolution limits of conventional lithography. Since conventional density multiplication techniques would affect the entire surface of the patterned media, these techniques may not be used.
In another example, integrated circuits often include various non-uniform and non-repeating features. Conventional density multiplication techniques, such as self-assembly of block copolymers, would not be useful in fabricating integrated circuits because the different regions on the surface of the integrated circuit would each potentially require a different feature density.